<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>数组扁平化</title>
</head>

<body>
  <script>
    let arr = [
      {
        id: 1,
        children: [
          {
            id: 11,
            children: []
          },
          { id: 12, }]
      },
      {
        id: 2,
        children: [
          {
            id: 21,
            children: []
          }
        ]
      }
    ]

    function flatten(arr) {
      return arr.reduce((result, item) => {
        return result.concat(
          item,
          Array.isArray(item.children) ? flatten(item.children) : []
        );
      }, []);
    }
    console.log(flatten(arr));
  </script>
</body>

</html>